In the conventional manufacture of lubricating base oils from petroleum feedstocks, fractions obtained from a crude oil and boiling in the desired lubricating base oil range (each range having a separate viscosity range) are separately treated with a suitable solvent to remove primarily undesired aromatic compounds present in the fractions and affecting the properties thereof. Such solvent extraction processes (using, for instance, furfural, N-methyl-2-pyrrolidone, phenol or sulphur dioxide as the extractant) produce lubricating oil raffinates and aromatic extracts.
A nonconventional approach to the preparation of lubricating base oils comprises the catalytic hydrotreatment of suitable feedstocks. The catalytic hydrogenation is normally carried out at rather severe conditions, e.g. at temperatures up to 500.degree. C., and hydrogen pressures up to 200 bar using hydrogenation catalysts such as molybdenum, chromium, tungsten, vanadium, platinum, nickel, copper, iron or cobalt either as such or in the form of their oxides and/or sulphides and either supported on a suitable carrier such as alumina or silica or unsupported. Lubricating base oils having a higher viscosity index are thus prepared as the amount of polycyclic compounds present is reduced substantially. Also, sulphur and nitrogen compounds present in the feedstock to be hydrogenated will be reduced to a very large extent, typically for more than 90%.
Normally, for paraffinic crudes as lube oil feedstocks, a dewaxing treatment is carried out after the solvent extraction process or the hydrogenation process to improve (i.e. to reduce) the pour point of the resulting lubricating base oil. Both solvent dewaxing and catalytic dewaxing can be applied. In the past acid treatments and/or clay treatments have been used to improve the resistance to oxidation of the product and to further improve the color and color stability of the product. Also, a rather mild hydrogenation (also referred to as hydrofinishing) of raffinates has often been applied in this context.
Combinations of various treatments have been suggested extensively in the art with a view to improving one or more properties of the lubricating base oil to be produced.
Also, the technique of blending different lubricating base oils which have been subjected to one or more (pre)-treatments in order to improve the oxidation stability of the resulting mixture can be used advantageously, has been described e.g. in British patent specification No. 2,024,852.
One of the problems still remaining in the manufacture of lubricating base oils from distillates, in particular waxy distillates, and/or deasphalted oils concerns the phenomenon of over-cracking. This may occur when the bulk of a raffinate obtained from solvent extraction is subjected to catalytic hydrotreatment: valuable products are lost as they are either exposed to rather severe hydroprocessing conditions to obtain base oils with the desired properties, or should not have been exposed at all since they already had the required properties. Not only are substantial amounts of useful products lost, also too much reactor volume has to be used.